90 FRESHWATER ANIMALS 



body of water, might slowly replace the other clones which com- 

 prise the population. 



In a population of a pond-inhabiting species the ability to 

 produce a relatively large number of viable resting eggs (em- 

 bryos) would have considerable selective value. For most 

 species sexual reproduction is a necessary prelude to the forma- 

 tion of the resting stage. [It is of interest to note in passing, how- 

 ever, that the mechanism for producing resting eggs partheno- 

 genetically has become firmly established at least once, and it 

 has probably arisen several times (Edmondson, 1955; Brooks, 

 1956).] In these pond populations it is to be expected that nat- 

 ural selection will favor the development and retention of high 

 sensitivity to the environmental conditions which trigger the 

 processes leading to the formation of males and to the production 

 of haploid eggs. A rapid and extensive conversion of the popula- 

 tion from uniparental to biparental reproduction would thus re- 

 duce the demands on a dwindling food supply as well as produce 

 the (usually) necessary resting stages. 



After this consideration of the reproductive peculiarities of 

 Daphnia we are now in a position to appreciate more fully the 

 factors that appear to be responsible for the difficulties encoun- 

 tered in the systematica of this genus. These are: 



1. The wide range of phenotypic variation due to the sensitiv- 

 ity of the developmental processes to environmental conditions. 



2. The frequent coexistence of flourishing populations of sev- 

 eral species in the same body of water. 



3. The introgression of genetic material between these coexist- 

 ing species. 



Phenotypic Variation. The phenotypic variation in Daphnia 

 is readily discernible as it changes the body shape. Figure 2 

 indicates some of the variants to be found in a pond-dwelling 

 species (Daphnia sch0dleri Sars). The form of any crustacean is 

 structurally determined by the form of its exoskeleton. The differ- 

 ences between A, B, and C of Fig. 2 are due to the different ex- 

 tent to which the exoskeleton of head, carapace, and shell spine 

 have grown. In the species illustrated here, as well as in all the 

 other members of the genus with which we are concerned in this 



